Rescue of a Dictyostelium discoideum mutant defective in cyclic nucleotide phosphodiesterase

A cAMP signaling model explains the benefit of maintaining two forms of phosphodiesterase in Dictyostelium

Starving Dictyostelium cells respond chemotactically to cell-generated waves of cyclic adenosine -3',5'- monophosphate (cAMP) that guide cell aggregation toward a signaling center. In this process, a large number of cells are recruited, resulting in the formation of aggregation territories that are essential for fruiting body formation. The enzyme PdsA phosphodiesterase (PDE), a crucial component of the signaling system, breaks down the external cAMP and can be either membrane-bound or secreted. The existence of two such forms is unusual in cell biology, and it remains to be determined why they have both been maintained through evolution. Here, using a model of the cAMP signaling system, I show that colonies can successfully organize into aggregates over a wider range of initial cell densities when both forms of PDE are present in an appropriately tuned ratio than when only a single form is present. The model indicates that membrane-bound PDE maintains aggregation-territory integrity in colonies with high initial cell density, whereas the secreted form is important for wave propagation at low cell densities. Thus, the ultimate retention of both forms can increase territory size. These findings have implications for other excitable media, including Ca(2+) propagation in cardiac cells and propagation of electrical excitation in nerve axons, since these systems have similar features of spatial nonuniform "release" and "degradation" of the relevant signals.

The group migration of Dictyostelium cells is regulated by extracellular chemoattractant degradation

Starvation of Dictyostelium induces a developmental program in which cells form an aggregate that eventually differentiates into a multicellular structure. The aggregate formation is mediated by directional migration of individual cells that quickly transition to group migration in which cells align in a head-to-tail manner to form streams. Cyclic AMP acts as a chemoattractant and its production, secretion, and degradation are highly regulated. A key protein is the extracellular phosphodiesterase PdsA. In this study we examine the role and localization of PdsA during chemotaxis and streaming. We find that pdsA(-) cells respond chemotactically to a narrower range of chemoattractant concentrations compared with wild-type (WT) cells. Moreover, unlike WT cells, pdsA(-) cells do not form streams at low cell densities and form unusual thick and transient streams at high cell densities. We find that the intracellular pool of PdsA is localized to the endoplasmic reticulum, which may provide a compartment for storage and secretion of PdsA. Because we find that cAMP synthesis is normal in cells lacking PdsA, we conclude that signal degradation regulates the external cAMP gradient field generation and that the group migration behavior of these cells is compromised even though their signaling machinery is intact.

Seven Dictyostelium discoideum phosphodiesterases degrade three pools of cAMP and cGMP

The Dictyostelium discoideum genome uncovers seven cyclic nucleotide PDEs (phosphodiesterases), of which six have been characterized previously and the seventh is characterized in the present paper. Three enzymes belong to the ubiquitous class I PDEs, common in all eukaryotes, whereas four enzymes belong to the rare class II PDEs that are present in bacteria and lower eukaryotes. Since all D. discoideum PDEs are now characterized we have calculated the contribution of each enzyme in the degradation of the three important pools of cyclic nucleotides: (i) extracellular cAMP that induces chemotaxis during aggregation and differentiation in slugs; (ii) intracellular cAMP that mediates development; and (iii) intracellular cGMP that mediates chemotaxis. It appears that each cyclic nucleotide pool is degraded by a combination of enzymes that have different affinities, allowing a broad range of substrate concentrations to be degraded with first-order kinetics. Extracellular cAMP is degraded predominantly by the class II high-affinity enzyme DdPDE1 and its close homologue DdPDE7, and in the multicellular stage also by the low-affinity transmembrane class I enzyme DdPDE4. Intracellular cAMP is degraded by the DdPDE2, a class I enzyme regulated by histidine kinase/phospho-relay, and by the cAMP-/cGMP-stimulated class II DdPDE6. Finally, basal intracellular cGMP is degraded predominantly by the high-affinity class I DdPDE3, while the elevated cGMP levels that arise after receptor stimulation are degraded predominantly by a cGMP-stimulated cGMP-specific class II DdPDE5. The analysis shows that the combination of enzymes is tuned to keep the concentration and lifetime of the substrate within a functional range.

DdPDE4, a novel cAMP-specific phosphodiesterase at the surface of dictyostelium cells

Dictyostelium discoideum cells possess multiple cyclic nucleotide phosphodiesterases that belong either to class I enzymes that are present in all eukaryotes or to the rare beta-lactamase class II. We describe here the identification and characterization of DdPDE4, the third class I enzyme of Dictyostelium. The deduced amino acid sequence predicts that DdPDE4 has a leader sequence, two transmembrane segments, and an extracellular catalytic domain that exhibits a high degree of homology with human cAMP-specific PDE8. Expression of the catalytic domain of DdPDE4 shows that the enzyme is a cAMP-specific phosphodiesterase with a K(m) of 10 microm; cGMP is hydrolyzed at least 100-fold more slowly. The full-length protein is shown to be membrane-bound with catalytic activity exposed to the extracellular medium. Northern blots and activity measurements reveal that expression of DdPDE4 is low during single cell stages and increases at 9 h of starvation, corresponding with mound stage. A function during multicellular development is confirmed by the phenotype of ddpde4(-) knock-out strains, showing normal aggregation but impaired development from the mound stage on. These results demonstrate that DdPDE4 is a unique membrane-bound phosphodiesterase with an extracellular catalytic domain regulating intercellular cAMP during multicellular development.

A cell-counting factor regulating structure size in Dictyostelium

Developing Dictyostelium cells form large aggregation streams that break up into groups of 0.2 x 10(5) to 1 x 10(5) cells. Each group then becomes a fruiting body. smlA cells oversecrete an unknown factor that causes aggregation streams to break up into groups of approximately 5 x 10(3) cells and thus form very small fruiting bodies. We have purified the counting factor and find that it behaves as a complex of polypeptides with an effective molecular mass of 450 kD. One of the polypeptides is a 40-kD hydrophilic protein we have named counting. In transformants with a disrupted counting gene, there is no detectable secretion of counting factor, and the aggregation streams do not break up, resulting in huge (up to 2 x 10(5) cell) fruiting bodies.

Null mutations of the Dictyostelium cyclic nucleotide phosphodiesterase gene block chemotactic cell movement in developing aggregates

Extracellular cAMP is a critical messenger in the multicellular development of the cellular slime mold Dictyostelium discoideum. The levels of cAMP are controlled by a cyclic nucleotide phosphodiesterase (PDE) that is secreted by the cells. The PDE gene (pdsA) is controlled by three promoters that permit expression during vegetative growth, during aggregation, and in prestalk cells of the older structures. Targeted disruption of the gene aborts development, and complementation with a modified pdsA restores development. Two distinct promoters must be used for full complementation, and an inhibitory domain of the PDE must be removed. We took advantage of newly isolated PDE-null cells and the natural chimerism of the organism to ask whether the absence of PDE affected individual cell behavior. PDE-null cells aggregated with isogenic wild-type cells in chimeric mixtures, but could not move in a coordinated manner in mounds. The wild-type cells move inward toward the center of the mound, leaving many of the PDE-null cells at the periphery of the aggregate. During the later stages of development, PDE-null cells in the chimera segregate to regions which correspond to the prestalk region and the rear of the slug. Participation in the prespore/spore population returns with the restoration of a modified pdsA to the null cells.

G alpha 3 regulates the cAMP signaling system in Dictyostelium

The Dictyostelium discoideum developmental program is initiated by starvation and its progress depends on G-protein-regulated transmembrane signaling. Disruption of the Dictyostelium G-protein alpha-subunit G alpha 3 (g alpha 3-) blocks development unless the mutant is starved in the presence of artificial cAMP pulses. The function of G alpha 3 was investigated by examining the expression of several components of the cAMP transmembrane signaling system in the g alpha 3- mutant. cAMP receptor 1 protein, cyclic nucleotide phosphodiesterase, phosphodiesterase inhibitor, and aggregation-stage adenylyl cyclase mRNA expression were absent or greatly reduced when cells were starved without exogenously applied pulses of cAMP. However, cAMP receptor 1 protein and aggregation-stage adenylyl cyclase mRNA expression were restored by starving the g alpha 3- cells in the presence of exogenous cAMP pulses. Adenylyl cyclase activity was also reduced in g alpha 3- cells starved without exogenous cAMP pulses compared with similarly treated wild-type cells but was elevated to a level twofold greater than wild-type cells in g alpha 3- cells starved in the presence of exogenous cAMP pulses. These results suggest that G alpha 3 is essential in early development because it controls the expression of components of the transmembrane signaling system.

Interacting signaling pathways controlling multicellular development in Dictyostelium

cAMP functions as the key extracellular signaling molecule controlling Dictyostelium development acting through classic G-protein-coupled/serpentine receptors. Whereas aggregation is controlled by nanomolar pulses of cAMP, a more continuous micromolar signal controls multicellular differentiation by activating a transcriptional cascade via a receptor-mediated but non G-protein-coupled pathway. Potential mechanisms by which extracellular cAMP functions to differentially control aggregation followed by morphogenesis and cell-type differentiation are discussed. This review also summarizes new findings elucidating pathways controlling cell-type regulation in this organism, including signaling cascades mediated by glycogen synthase kinase 3 and cAMP-dependent protein kinase, key regulators of cell-type differentiation in metazoans, and newly identified transcription factors.

A Dictystelium mutant with defective aggregate size determination

Starved Dictyostelium cells aggregate into groups of roughly 10(5) cells. We have identified a gene which, when repressed by antisense transformation or homologous recombination, causes starved cells to form large numbers of small aggregates. We call the gene smlA for small aggregates. A roughly 1.0 kb smlA mRNA is expressed in vegetative and early developing cells, and the mRNA level then decreases at about 10 hours of development. The sequence of the cDNA and the derived amino acid sequence of the SmlA protein show no significant similarity to any known sequence. There are no obvious motifs in the protein or large regions of hydrophobicity or charge. Immunofluorescence and staining of Western blots of cell fractions indicates that SmlA is a 35x10(3) Mr cytosolic protein present in all vegetative and developing cells and is absent from smlA cells. The absence of SmlA does not affect the growth rate, cell cycle, motility, differentiation, or developmental speed of cells. Synergy experiments indicate that mixing 5% smlA cells with wild-type cells will cause the wild-type cells to form smaller fruiting bodies and aggregates. Although there is no detectable SmlA protein secreted from cells, starvation medium conditioned by smlA cells will cause wild-type cells to form large numbers of small aggregates. The component in the smlA-conditioned media that affects aggregate size is a molecule with a molecular mass greater than 100x10(3) Mr that is not conditioned media factor, phosphodiesterase or the phosphodiesterase inhibitor. The data thus suggest that the cytosolic protein SmlA regulates the secretion or processing of a secreted factor that regulates aggregate size.

Dual role of cAMP during Dictyostelium development

cAMP plays an essential role during Dictyostelium development both outside and inside the cell. Membrane-bound receptors and adenylyl cyclase are responsible for sensing and producing extracellular cAMP, whereas a phosphodiesterase is responsible for maintaining a low basal level. The molecular events underlying this type of hormone like signalling, which are now beginning to be deciphered, will be presented, in the light of cAMP analogue studies. The importance of intracellular cAMP for cell differentiation has been demonstrated by the central role of the cAMP dependent protein kinase. Mutants as well as strains obtained by reverse genetics will be reviewed which lead to our current understanding of the role of intracelluar cAMP in the differentiation of both stalk and spore cells.

V4, a gene required for the transition from growth to development in Dictyostelium discoideum

The V4 gene of Dictyostelium discoideum is regulated in a nutrient-dependent manner and is deactivated immediately upon the onset of development. V4 is expressed only during growth, but its expression is not required for growth. We propose that the V4 gene product plays a role in the transition from growth to development. We have tested this hypothesis by antisense mutagenesis. Cells transformed with a V4 antisense construct contained no detectable endogenous V4 mRNA. These cells grew normally, but they failed to aggregate. Under conditions which normally promote development, V4 antisense transformants failed to deactivate vegetative-specific genes. These cells also were unable to induce the expression of the cAMP cell surface receptor, the cyclic nucleic phosphodiesterase, and contact sites A, all of which are normally induced under such conditions. Surprisingly, cells transformed with a V4 sense construct displayed a similar morphological and biochemical phenotype as the antisense cells, whereas cells transformed with the parental vector exhibited a normal biochemical and morphological phenotype. These results demonstrate that expression of the V4 gene during growth is required for the proper initiation of development.

cAMP signal transduction pathways regulating development of Dictyostelium discoideum

Dictyostelium discoideum development is regulated through receptor/G protein signal transduction using cAMP as a primary extracellular signal. Signaling pathways will be discussed as well as the regulation and function of individual cAMP receptors and G alpha subunits. Finally potential downstream targets including protein kinases and nuclear events will be explored.

Cyclic nucleotide phosphodiesterase of Dictyostelium discoideum and its glycoprotein inhibitor: structure and expression of their genes

The genes coding for the cyclic nucleotide phosphodiesterase (PD) and the PD inhibitory glycoprotein (PDI) have been cloned and characterized. The PDI gene was isolated as a 1.6 kb genomic fragment, which included the coding sequence containing two small introns and 510 nucleotides of non-translated 5' sequence. From the deduced amino acid sequence we predict a protein with a molecular weight (MW) of 26,000 that, in agreement with previous data, contains 15% cysteine residues. Genomic Southern blot analysis indicates that only one gene encodes the inhibitor. Northern blot analysis shows a single transcript of 0.95 kb. The PDI gene is expressed early in development with little transcript remaining following aggregation. The appearance of PDI mRNA is prevented by the presence of cAMP, but when cAMP is removed the transcript appears within 30 minutes. When cAMP is applied to cells expressing PDI the transcript disappears with a half-life of less than 30 minutes. The PD gene of D. discoideum is transcribed into three mRNAs: a 1.9 kb mRNA specific for growth, a 2.4 kb mRNA specific for aggregation, and a 2.2 kb mRNA specific for late development. The 2.2 kb mRNA is also specific for prestalk cells, and is induced by differentiation-inducing factor. All three mRNAs contain the same coding sequence, and differ only in their 5' non-coding sequences. Each mRNA is transcribed from a different promoter, and by using the chloramphenicol acyltransferase gene as a reporter, we have shown that each promoter displays the same regulation as its cognate mRNA.(ABSTRACT TRUNCATED AT 250 WORDS)

A developmentally regulated and cAMP-repressible gene of Dictyostelium discoideum: cloning and expression of the gene encoding cyclic nucleotide phosphodiesterase inhibitor

A 1.6-kb genomic fragment containing the coding region for the inhibitor (PDI) of cyclic nucleotide phosphodiesterase (PD) was isolated and sequenced. The genomic sequence includes 510 nucleotides (nt) of 5'-noncoding sequence and the full coding sequence, which contains two small introns. From the deduced amino acid (aa) sequence we predict a 26-kDa protein that, in agreement with previous data, contains approximately 15% Cys residues. The PDI possesses a hydrophobic leader sequence, five potential glycosylation sites, and three internal repeats. Northern-blot analysis showed a single transcript of 0.95 kb. The gene encoding PDI (pdi) was expressed early in development with little transcript remaining following aggregation. The appearance of pdi transcript was inhibited by cAMP, but when cAMP was removed the transcript appeared within 30 min. When cAMP was applied to cells containing pdi mRNA, the transcript disappeared with a half-life of less than 30 min.

The cyclic nucleotide phosphodiesterase gene of Dictyostelium discoideum contains three promoters specific for growth, aggregation, and late development

The cyclic nucleotide phosphodiesterase (phosphodiesterase) plays essential roles throughout the development of Dictyostelium discoideum. It is crucial to cellular aggregation and to postaggregation morphogenesis. The phosphodiesterase gene is transcribed into three mRNAs, containing the same coding sequence connected to different 5' untranslated sequences, that accumulate at different times during the life cycle. A 1.9-kilobase (kb) mRNA is specific for growth, a 2.4-kb mRNA is specific for aggregation, and a 2.2-kb mRNA is specific for late development and is only expressed in prestalk cells. Hybridization of RNA isolated from cells at various stages of development with different upstream regions of the gene indicated separate promoters for each of the three mRNAs. The existence of specific promoters was confirmed by fusing the three putative promoter regions to the chloramphenicol acetyltransferase reporter gene, and the analysis of transformants containing these constructs. The three promoters are scattered within a 4.1-kilobase pair (kbp) region upstream of the initiation codon. The late promoter is proximal to the coding sequence, the growth-specific promoter has an initiation site that is 1.9 kbp upstream of the ATG codon, and the aggregation-specific promoter has an initiation site 3 kbp upstream.

The cyclic nucleotide phosphodiesterase gene of Dictyostelium discoideum contains three promoters specific for growth, aggregation, and late development

The cyclic nucleotide phosphodiesterase (phosphodiesterase) plays essential roles throughout the development of Dictyostelium discoideum. It is crucial to cellular aggregation and to postaggregation morphogenesis. The phosphodiesterase gene is transcribed into three mRNAs, containing the same coding sequence connected to different 5' untranslated sequences, that accumulate at different times during the life cycle. A 1.9-kilobase (kb) mRNA is specific for growth, a 2.4-kb mRNA is specific for aggregation, and a 2.2-kb mRNA is specific for late development and is only expressed in prestalk cells. Hybridization of RNA isolated from cells at various stages of development with different upstream regions of the gene indicated separate promoters for each of the three mRNAs. The existence of specific promoters was confirmed by fusing the three putative promoter regions to the chloramphenicol acetyltransferase reporter gene, and the analysis of transformants containing these constructs. The three promoters are scattered within a 4.1-kilobase pair (kbp) region upstream of the initiation codon. The late promoter is proximal to the coding sequence, the growth-specific promoter has an initiation site that is 1.9 kbp upstream of the ATG codon, and the aggregation-specific promoter has an initiation site 3 kbp upstream.

Complementation of myosin null mutants in Dictyostelium discoideum by direct functional selection

The eukaryotic slime mold Dictyostelium discoideum contains a single conventional myosin heavy chain gene (mhcA). Cell lines in which this gene was deleted via homologous recombination have been previously reported. These myosin null cells were shown to be defective for cytokinesis and for sporogenesis. We demonstrate here that the cloned mhcA gene can be reintroduced into these cells by the use of a direct functional selection. This selection was imposed by demanding that cells be capable of growth in suspension. The resulting transformants appear normal for cytokinesis, and also are fully competent for sporogenesis, confirming that reintroduction of the myosin gene is sufficient to restore these properties. These results demonstrate a method for rescuing mutants in Dictyostelium which may be generally applicable for genetically created mutations as well as for mutations which have been engineered.

The cyclic nucleotide phosphodiesterase gene of Dictyostelium discoideum utilizes alternate promoters and splicing for the synthesis of multiple mRNAs

The cyclic nucleotide phosphodiesterase (phosphodiesterase) gene plays essential roles in the development of Dictyostelium discoideum during cellular aggregation and postaggregation morphogenesis. Genomic clones spanning the gene were isolated and used to determine the sequence and structure of the phosphodiesterase gene. We found an unusually complex organization for a gene of D. discoideum. Two transcripts of 2.4 and 1.9 kilobases (kb) were synthesized from start sites separated by 1.1 kb. A developmentally regulated promoter was utilized for the 2.4-kb mRNA, and a constitutive promoter regulated synthesis of the 1.9-kb transcript. The gene was found to be divided into four exons that are alternately spliced to give rise to the two mRNAs. The precursor of the 2.4-kb mRNA contained a 2.3-kb intron, whereas the precursor of the constitutive transcript was synthesized with a 1.7-kb intron. The two transcripts contained identical protein-coding regions and 400-nucleotide 3' untranslated sequences. The 2.4-kb developmentally regulated mRNA was distinguished by a long 5' untranslated leader of 666 nucleotides. The complex structure of the gene may allow multiple levels of control of the expression of the phosphodiesterase during development.

Dictyostelium discoideum: a model system for cell-cell interactions in development

The cellular slime mold Dictyostelium discoideum undergoes a transition from single-celled amoebae to a multicellular organism as a natural part of its life cycle. A method of cell-cell signaling that controls chemotaxis, morphogenesis, and gene expression has developed in this organism, and a detailed understanding of this signaling system provides clues to mechanisms of intercellular communication in the development of metazoans.

The cyclic nucleotide phosphodiesterase gene of Dictyostelium discoideum utilizes alternate promoters and splicing for the synthesis of multiple mRNAs

The cyclic nucleotide phosphodiesterase (phosphodiesterase) gene plays essential roles in the development of Dictyostelium discoideum during cellular aggregation and postaggregation morphogenesis. Genomic clones spanning the gene were isolated and used to determine the sequence and structure of the phosphodiesterase gene. We found an unusually complex organization for a gene of D. discoideum. Two transcripts of 2.4 and 1.9 kilobases (kb) were synthesized from start sites separated by 1.1 kb. A developmentally regulated promoter was utilized for the 2.4-kb mRNA, and a constitutive promoter regulated synthesis of the 1.9-kb transcript. The gene was found to be divided into four exons that are alternately spliced to give rise to the two mRNAs. The precursor of the 2.4-kb mRNA contained a 2.3-kb intron, whereas the precursor of the constitutive transcript was synthesized with a 1.7-kb intron. The two transcripts contained identical protein-coding regions and 400-nucleotide 3' untranslated sequences. The 2.4-kb developmentally regulated mRNA was distinguished by a long 5' untranslated leader of 666 nucleotides. The complex structure of the gene may allow multiple levels of control of the expression of the phosphodiesterase during development.